Method for inducing a volumetric change in a nanoporous material

ABSTRACT

The invention is directed to a method for inducing a surface stress-induced volumetric change in a nanoporous material and to a device for carrying out the method. The method of the invention comprises—accumulating polar molecules onto the surface of the nanoporous material by physical adsorption of the polar molecules from the vapour phase thereby inducing surface stress to said surface; or—dissipating accumulated polar molecules from the surface of the nanoporous material by physical desorption of the polar molecules into the vapour phase thereby inducing surface stress to said surface, wherein the nanoporous material comprises a nanoporous metal or alloy. The device comprises a) a nanoporous metal material which, upon physical adsorption from the vapour phase or physical desorption into the vapour phase of polar molecules onto or from the surface of the nanoporous metal material, exhibits a volumetric change; and b) a mechanism for detecting and/or transferring the volumetric change.

The invention is directed to a method for inducing a volumetric change in a nanoporous material and to a device for carrying out the method.

Nowadays progress in technical applications is dominated by miniaturisation of devices. In view of this miniaturisation, new mechanisms and solutions are needed, for instance, to efficiently perform mechanical work at micrometer and nanometer scales. Considerable research effort has been put into finding such new mechanisms and solutions.

These efforts have resulted in a number of ways to induce changes in the macroscopic dimension of materials by controlling small macroscopic strains.

Lee et al. (Nanotechnology 2008, 19(16), 165501), for example, describe electrochemically-induced volumetric changes in electroactive polymers at the nanoscale by using individual polypyrrole nanowires with diameters under 100 nm.

Some of the known concepts include charge-induced strains in nanoporous metals, surface-chemistry-driven actuation in nanoporous metals, fuel-powered artificial muscles, carbon nanotube actuators, molecular actuators, conducting polymer actuators, ionic polymer metal composites, dielectric elastomers, and relaxor ferroelectric polymers.

EP-A-2 050 954 describes a method for controlling macroscopic strain of a nanoporous metal structure, wherein a modifying agent is chemically adsorbed onto the surface of the nanoporous metal. It is stated in this patent application that covalent adsorbate-metal interactions play a decisive role. Therefore, the invention of EP-A-2 050 954 is directed to a surface chemistry driven actuation, thus requiring chemical reaction energy. In addition, the method of EP-A-2 050 954 requires the use of a modifying agent for modifying the surface stress of the porous structure and a removing agent for removing the chemically adsorbed modifying agent from the porous structure in order to invoke a volumetric recovery. For some applications, this combined use of modifying agent and removing agent is undesirable.

Existing methods to control small macroscopic strains, however, rely upon external energy sources in the form of electricity, heat, or chemical reaction energy. It would be desirable to control macroscopic displacements without the need for these external energies.

In this respect Okuzaki et al. (Journal of Applied Polymer Science 1997, 64(2), 383-388) studied dimensional changes of polypyrrole in response to adsorption of polar vapour molecules. Polypyrrole was found to expand on physical adsorption of water vapour.

Biener et al. (Nature Materials 2009, 8, 47-51) describes surface-chemistry-driven actuation in nanoporous gold. Reversible strain amplitude is achieved by alternating exposure of nanoporous gold to ozone an carbon monoxide. Ozone exposure leads to chemical oxygen adsorption on the Au surface (O₃+Au→Au—O+O₂), while carbon monoxide exposure results in the chemically adsorbed oxygen to react with the carbon monoxide to form carbon dioxide and simultaneously restoring the Au surface (CO+Au—O→CO₂+Au). Although the molecules are initially physically adsorbed onto the gold surface, this process does not involve surface stress-induced volumetric changes at the early stage of the chemical adsorption process because during chemical adsorption, molecules that reach the surface immediately react to form other substances. By the time the next molecules arrive onto the surface, the previous ones are no longer physically adsorbed on the surface, because they have already reacted with the surface. Hence, there is no accumulation of physically pre-adsorbed molecules on the surface during chemical adsorption. The volumetric change in Biener et al. is caused by the atoms that remain chemically bonded onto the surface. The reaction of ozone molecules (O₃) onto the gold surface results in the formation of oxygen molecules (O₂) and oxygen atoms (O). During this process, ozone molecules that reach the surface do not accumulate onto that gold surface: they are immediately converted into oxygen molecules, which do not remain on the surface, and oxygen atoms, which remain chemically bonded onto the surface.

Objective of the invention is therefore to provide a method for creating a volumetric change without the need of an external energy source in the form of electricity, heat or chemical reaction energy.

The inventors found that this objective can, at least in part, be met by utilising physical adsorption or physical desorption of polar molecules onto or from a nanoporous metal or alloy. Accordingly, the invention relates to a method for creating a volumetric change in a nanoporous material, comprising

-   -   physically adsorbing polar molecules from the vapour phase onto         the surface of the nanoporous material; or     -   physically desorbing polar molecules from the surface of the         nanoporous material into the vapour phase,         wherein the nanoporous material comprises a nanoporous metal or         alloy.

More in particular, in a first aspect the invention is directed to a method for creating a surface stress-induced volumetric change in a nanoporous material, comprising

-   -   accumulating polar molecules onto the surface of the nanoporous         material by physical adsorption of the polar molecules from the         vapour phase thereby inducing surface stress to said surface; or     -   dissipating accumulated polar molecules from the surface of the         nanoporous material by physical desorption of the polar         molecules into the vapour phase thereby inducing surface stress         to said surface,         wherein the nanoporous material comprises a nanoporous metal or         alloy.

The use of adsorption/desorption of polar molecules was surprisingly found to induce a considerable volumetric change in the nanoporous material. Advantageously, the method of the invention does not require the above-mentioned external energy sources for creating the volumetric change in the nanoporous material. Rather, it makes use of energy that arises from the liquid-gas phase transition of the polar molecules.

The term “nanoporous” as used in this application is meant to refer to a material that includes a significant number of voids with a dimension, e.g. an average cross-sectional diameter, in the range of 1 nm to 1000 nm. The void dimension can be measured by scanning electron microscopy.

The term “physical adsorption” as used in this application is meant to refer to the process where the polar molecules bind to the surface of the nanoporous material via weak intermolecular forces, such as Van der Waals forces or induced dipole forces.

The term “physical desorption” as used in this application is meant to refer to the reverse process of physical adsorption.

The volumetric change which is achieved by the method of the invention can be an increase or decrease in one or more dimensions. Typically, a volumetric change includes an expansion or contraction.

A key feature of nanoporous materials is the high surface-to-volume ratio, which causes variations in the surface stress. Without wishing to be bound by theory, the inventors believe that the volumetric changes achieved by the method of the invention are the result from changes in the surface stress of the nanoporous material. It is believed that the surface stress is affected by physical adsorption or physical desorption of molecules onto or from the nanoporous material. In metals and alloys, the creation of a new surface gives rise to unbalanced bonds at this surface. These unbalanced electric charges redistribute so as to reduce the equilibrium bond length between the metal surface atoms. For a clean metal surface, charge redistribution gives rise to a tensile surface stress. In the method of the invention, it is believed that the polar molecules adsorb onto the surface of the nanoporous material with a specific part of the molecule due to their polarity. This induces a temporary electric field at the surface of the nanoporous material. In response to the induced electric field, the unbalanced charges in the nanoporous material tend to relocate from the surface to the bulk, which generates an increase in tensile surface stress. During physical adsorption, the adsorbate molecules gradually accumulate onto the metal surface. The more molecules are accumulated, the higher their interaction with the surface (Van der Waals).

Due to the high surface-to-volume ratio of the nanoporous material, an increase in tensile surface stress gives rise to a macroscopic shrinkage of the material. During desorption, the polar molecules leave the surface and the induced dipole fields vanish. Charge redistribution occurs and the tensile surface stress decreases, thereby resulting in macroscopic relaxation of the specimen.

This mechanism giving rise to the a surface stress-induced volumetric change is fundamentally different from, for example, volume expansion due to a filling of the pores (capillary condensation) of the nanoporous material with water (such as by exposing the nanoporous material to water vapour). In this different mechanism, nanoporous materials (either metals or not) can undergo volume expansion when their pores are filled with water. This swelling effect is different from the surface stress-induced volumetric changes that are subject of the present invention. For example, a nanoporous material always expands when the pores are filled with adsorbate molecules, while a nanoporous metal does not necessarily expand when its surface adsorbs foreign elements. It may also contract upon adsorption of adsorbate molecules, depending on the electronegativity of the surface and/or the adsorbed elements.

In addition to the possible difference in sign, another pronounced distinction between the swelling effect and the mechanism of physical adsorption-induced volumetric changes includes the quantity of adsorbates required to generate volumetric changes. For instance, depending on the pores geometry, the swelling effect may become detectable when at least the half of the pores volume fraction is filled with water. In contrast, the physical adsorption induced-dimensional changes is associated to the coverage of the pores surface with a sub-monolayer of adsorbates, or preferably a monolayer of adsorbates, or more generally filling less than 10% volume fraction of the pores with adsorbates.

Hence, in a preferred embodiment said accumulating comprises the physical adsorption of at least a monolayer of the polar molecules onto the surface of the nanoporous material. The accumulation preferably leads to the nanoporous material being filled with polar molecules to less than 10 vol. % of the total pore volume of the nanoporous material. Similarly, in a preferred embodiment said dissipating comprises the physical desorption of at least a monolayer of the accumulated polar molecules from the surface of the nanoporous material. The dissipation preferably leads to the nanoporous material being depleted in accumulated polar molecules by less than 10 vol. % of the total pore volume of the nanoporous material.

In accordance with the invention, the nanoporous material comprises a nanoporous metal or alloy. It is preferred that the nanoporous metal or alloy comprises a noble metal, preferably selected from the group consisting of gold, platinum, palladium and silver. Noble metals are preferred, because they do not oxidise under standard atmospheric conditions and allow a good performance in extreme environments (such as environments that contain chemical vapours). Furthermore, it is preferred that the nanoporous material is hydrophilic. The nanoporous material can further comprise impurities and/or an oxide layer.

The nanoporous material preferably has a high surface-to-volume ratio. High surface-to-volume ratios give rise to a strong increase in tensile surface stress upon physical adsorption of the polar molecules. The nanoporous material preferably has a surface-to-volume ratio of 10 m²/cm³ or more, more preferably 25 m²/cm³ or more, and even more preferably in the range of 25-500 m²/cm³, such as in the range of 25-200 m²/cm³. The surface-to-volume ratio of a nanoporous material can be determined by taking the product of its specific surface area (i.e. surface area per unit mass) to its density (i.e. mass per unit of volume). Preferably, this surface-to-volume ratio is the metal surface-to-volume ratio. The presence of non-metallic compounds in a composite material reduces the metal surface-to-volume ratio of the composite material. In metal-organic frameworks, for instance, metallic clusters are interconnected through organic ligands. The fraction of the metal content to the total metal-organic framework volume is not well-quantified since there are various types of metal-organic frameworks. However, the size of the organic linkers (typically polymers chains) in metal-organic frameworks is larger than that of the metal clusters. Accordingly, the metal surface-to-volume fraction in metal-organic frameworks is low as compared to the surface-to-volume ratio of pure nanoporous metals (or alloys). Hence, in a preferred embodiment, the nanoporous material consists of nanoporous metal or alloy.

The porosity of the nanoporous material can be in the range of from 0.3 or more. The term porosity, as used herein, is defined as the proportion of non-solid volume to the total volume (i.e. non-solid and solid) of the nanoporous material. Preferably, the porosity is in the range of from 0.3 to 0.95, such as in the range of from 0.4 to 0.9. The porosity is unity minus the nanoporous material relative density. Here the relative density is the ratio of the density of the nanoporous material to the density of the material in the bulk solid form.

In accordance with the invention, the metal or alloy is nanoporous. As defined herein, a nanoporous material has voids with a dimension in the range of 1 nm to 1000 nm. The nanoporous materials can be a material wherein the pores have an average cross-sectional diameter as measured by scanning electron microscopy of 500 nm or less, preferably 100 nm or less, more preferably in the range of 2-60 nm.

Methods for preparing a nanoporous metal or alloy material are commonly known in the art, including e.g. dealloying which is defined as selective corrosion (removal) of the less noble constituent from an alloy, usually via dissolving this component in a corrosive environment. For example, nanoporous gold can be prepared by selectively leaching silver from an Ag—Au alloy using either a strong oxidising acid such as nitric acid (free corrosion) or by applying an electrochemical driving force (electrochemically-driven dealloying). Both methods lead to the development of a nanoporous open-cell morphology. In the case of silver-gold alloys, this technique leads to the development of a three-dimensional bicontinuous nanoporous structure while maintaining the original shape of the alloy sample. Chemical analysis of the material reveals that almost pure gold may be achieved using this process.

In an embodiment, the nanoporous material may be applied onto a carrier material. The carrier material may be the same as the nanoporous material, but can also be a different material. The carrier material can be a porous (such as a nanoporous) material or a non-porous material. In an embodiment, the nanoporous material is a nanoporous layer that is applied onto a chemically different material. This chemically different material is preferably a metal or alloy, but may also be a different material. In a further embodiment, the nanoporous material is one layer of a multi-layer foil, such as a nanoporous metal layer in a bi-metal layer foil. The nanoporous material reinforces the stress gradient in the multi-layer foil. This is a way to amplify strain amplitude.

The polar molecules can comprise one or more selected from polar organic solvents including polar protic and polar aprotic solvents. Suitable polar molecules include water, methanol, and ethanol. Preferably, the polar molecules comprise water molecules.

It is preferred that the physical adsorption or desorption is the result of a change in vapour pressure of the polar molecules in the environment. In an embodiment, the physical adsorption or desorption is the result of a change in relative humidity of the environment. This is particularly advantageous when the method is used, e.g. in a sensing device.

Suitably, the volumetric change achieved with the method of the invention is, at least partially, reversible. By stating that the volumetric change is at least partially reversible, it is intended that the nanoporous material may substantially return to its former volume when after physical adsorption of polar molecules, the polar molecules are physically desorbed, and vice versa.

Whether the volumetric change of the nanoporous material upon adsorption is a contraction or an expansion depends on i) the nature (e.g. polarity sign) of the adsorbed molecules (polar protic molecules give a different response than polar aprotic molecules), and ii) the state of the adsorbing surface (a clean surface behaves differently than an oxide-covered metal surface). These insights can be used as a tool to design the desired response for various applications.

In an embodiment of the invention, the volumetric change is a contraction when polar molecules physically adsorb onto the surface of the nanoporous material, and the volumetric change is an expansion when polar molecules physical desorb from the surface of the nanoporous material.

The method of the invention can achieve a volumetric change which is characterised by a strain amplitude of 0.005% or more, preferably 0.01% or more in response to a change in vapour pressure of the polar molecules in the environment of 15%. This strain amplitude is sufficient for the method of the invention to be used in a wide variety of applications.

In a further aspect, the invention is directed to a device for carrying out the method of the invention, said device comprising

-   a) a nanoporous metal or alloy material which, upon physical     adsorption from the vapour phase or physical desorption into the     vapour phase of polar molecules onto or from the surface of the     nanoporous metal material, exhibits a volumetric change; and -   b) a mechanism for detecting and/or transferring the volumetric     change.

The mechanism for detecting and/or transferring the volumetric change can, for instance, be a piston/displacement unit, a mechanical lever, an optical sensor, an electrical switch, etc.

The device of the invention can suitably be a sensing device or an actuating device. Preferably, the device of the invention is operative in the absence of electricity, heat, and chemical reaction energy external stimuli.

In a preferred embodiment, the nanoporous metal or alloy material has a layered structure wherein each layer comprises a multitude of scales, and wherein at least a part of the scales in each layer are locally attached to one or more scales of an adjacent layer. Since each of the layers comprises a multitude of scales that are not necessarily connected, each layer can be a non-continuous layer. The scales may be partially delaminated. The average thickness of the layers can vary and is typically in the range of 10-500 nm, preferably 20-400 nm, such as 50-200 nm. The size of the scales in terms of an equivalent spherical diameter can, for instance, be from 1-500 μm, preferably 5-400 μm, such as 10-200 μm.

When a device according to this preferred embodiment undergoes a volumetric change, the scales open by bending-buckling, because they are locally clamped onto the underlying layers. The device of this preferred embodiment is suitable for adsorption of foreign molecules by physical adsorption according to the method of the invention, but may also be applied for creating a volumetric change by chemical adsorption of atoms or by electro-adsorption of ions.

Advantageously, this preferred device can show volumetric changes with a strain amplitude of 3-6% or even higher.

The device of the invention is particularly useful in the semiconductor industry. More particular, the device of the invention can be used as a sensor and/or actuator. Sensors and/or actuators that comprise the device of the invention have various applications, including humidity sensors, impurity sensors (in particular sensors that can specifically detect polar molecules), smart systems (e.g. for removing polar molecules from the air), displacement control (such as a switch for chip making industry or a device to enter a cell by pulsing through the cell membrane), and porosity based systems (such as a filtering system, or an anchor system for cell matrix in prostheses).

The sensitivity of the device of the invention when used as a sensing device is very high, since the sensitivity is associated to the ability of the material to adsorb foreign molecules. The sensitivity can be improved by increasing the specific surface of the nanoporous material, which can be done, e.g. by simply synthesising nanoporous materials with smallest ligament diameters and pore sizes possible.

For humidity sensor applications, it was observed that the device of the invention responds to very small changes in relative humidity. For example, for nanoporous gold specimens it was found that the response was better than the capacitive humidity sensor (accuracy of 2%) used to probe changes in relative humidity during the experiments. There is a strong demand for good humidity detection, for instance, in vacuum chambers, high tech clean-rooms, and pharmaceutical and biotechnological facilities.

Displacement control devices can be used for precision work on the micrometer to nanometer scale. As an example, a device used to enter a cell can be mentioned. So far, piezo-electrical systems are used for these kind of applications. The device of the invention allows a smoother control of the displacement and lowers the possibility of damage, such as damage to a cell.

In a special aspect, the invention is directed to a method for delivering an active compound, which comprises the step of including an active compound in a nanoporous metal or alloy as defined herein. The active compound is included in the nanoporous material. This may, for instance, involve expanding the nanoporous metal or alloy material by a surface-induced volumetric change. Next, active compound can be captured in the nanoporous material, e.g. by applying a surface-induced volumetric change, such as a compression of the nanoporous material. When delivered at a pre-determined location, the active compound may be released from the nanoporous material, again by inducing a surface-induced volumetric change. The surface-induced volumetric changes may be the result of changes of the environment of the nanoporous metal or alloy material. The preferred high surface-to-volume ratio of the nanoporous material used in the invention is highly advantageous for the inclusion of active compounds. Possible active compounds can for example include antibodies for biomedical applications. The mechanism can, for instance, be explored to introduce antibodies in porous metal nanoparticles and deliver these antibodies at specific locations.

The invention is further illustrated by the following Examples, which are not intended to be limiting in any way.

EXAMPLES Nanoporous Gold Synthesis

Nanoporous gold specimens were synthesised by an electrochemically selective dissolution of silver from a gold-silver alloy with compositions Au₂₅Ag₇₅ (atom %). The master alloy was prepared by arc melting of pure gold (99.99%) and silver (99.999%). After homogenisation in a quartz tube at 950° C. for 56 hours rectangular blocks with dimensions of 2×1×1 mm³ were cut by spark erosion and annealed at 850° C. for 3 hours. Subsequently, all silver was removed by selective dissolution in 1 M perchloric acid. Silver dissolution was controlled by a potential of 850 mV measured against an Ag/AgCl reference electrode in a three-electrode electrochemical cell. Nanoporous gold speciments with ligaments diameter of about 20 nm (as determined by scanning electron microscopy) and porosity up to 77% (evaluated from the nanoporous material relative density which was between 23% and 26%) were obtained from these blocks. FIG. 1 displays a scanning electron micrograph of the nanoporous gold structure. Due to its high surface-to-volume ratio (about 75 m²/cm³, which is the product of the nanoporous material density to its specific surface area), the nanoporous gold is very sensitive to variations in the surface stress.

Capillary Pressure in the Pores

Beside a disjoining pressure which is associated to the coverage of the ligaments surface by a water film, capillary pressure may arise during meniscus formation when the pores are filled with water. In order to investigate the contribution of these capillary pressures, the total water content inside the pores was evaluated by thermogravimetric analysis (TGA). The maximum water content inside each specimen was found to vary between about 70 μg and about 90 μg. This corresponds to about 5.8% of the total void space. The low water content suggests that the pores are not filled, therefore capillary pressures are negligible. The total water content inside the pores do not contribute to the reversible dimension changes, since a water fraction is always present in the pores even at lowest relative humidity and can only be removed by high temperature treatment.

Strain Measurements

A profilometer (Mahr Perthometer S2/GD 25) having a contact force of 1 mN, with a vertical resolution of 1 nm was used in stationary mode for in situ measurements of the sample expansions and contractions. A thermo-hygrometer (FHA646-E1C Ahlborn) with accuracies of 0.1° C. for the temperature and 2% for the relative humidity was used to probe temperature and relative humidity fluctuations during measurements (for changes in relative humidity, see FIGS. 2A and 2B, upper panels). Saturated salts of lithium chloride (11.3% relative humidity at 20° C.) and potassium nitrate (94.6% relative humidity at 20° C.) were used in alternation to decrease and increase the equilibrium humidity during experiments. Recorded temperature fluctuations during experiments were all below 0.04° C. The expected thermal expansion caused by these small temperature fluctuations are on the order of 10⁻⁵%. This is three orders of magnitude lower than the measured strain, and thus negligible.

Water Film Thickness in the Pores

In situ measurements of isothermal changes in mass of a nanoporous gold specimen in response to a 20% change in relative humidity was performed using an ultra-precise microbalance. Changes in mass occur when a specimen is brought from a relative humidity of about 47% to a lower relative humidity of about 25% in the measuring chamber of the microbalance. FIG. 3 displays measurements on mass changes performed independently on three different specimens (left ordinate) and dimension changes of a specimen under similar conditions (right ordinate). The recorded loss of mass after ten minutes is up to about 30 μg. The corresponding water film thickness deduced from the specimen specific surface is about 0.3 nm (corresponding to about 1 monolayer of water).

Water Vapour-Induced Volumetric Changes

During physical adsorption, the nanoporous gold ligaments surface was covered with polar water vapour molecules from the ambient air. This process does not involve chemical reactions. Although the enthalpy of condensation (−40.7 kJ·mol⁻¹) and evaporation (40.7 kJ·mol⁻¹) of water are equal, a change in the Gibbs free energy of water occurs during physical adsorption. That change in the Gibbs free energy which is related to the loss/gain of entropy during condensation/evaporation, is the input energy for actuations. At standard atmospheric conditions, water molecules are characterised by their dynamic equilibrium between the liquid and gas states. When the relative humidity is increased or decreased at constant temperature a phase transition occurs and adsorption or desorption takes place at the gold surface due to the permanent dipole moment of water molecules. Note that that under standard atmospheric conditions, other gases present in air like N₂, O₂, Ar, CO₂ are not physically adsorbed because contrary to water vapour, their molecules are non-polar and their liquid-gas equilibrium lines are associated with high pressures and/or low temperatures.

Investigations of water vapour-induced dimensional changes in nanoporous gold specimens were carried out in a glove box maintained on an optical table to optimize the stability of the measuring setup. Changes in relative humidity versus time are shown in the upper panels of FIGS. 2A and 2B for short times exposure (about 30 minutes per cycle) and long times exposure (about 300 minutes per cycle), respectively. The corresponding strains versus time when the sample is alternately exposed to dry and humid air are shown in the lower panels of FIGS. 2A and 2B. Nanoporous gold is observed to contract upon water vapour adsorption and expand upon water vapour desorption. For long exposure times, reversible strain amplitudes up to 0.02% were measured in response to 15% changes in air relative humidity.

Strain versus relative humidity for short exposure times and long exposure times are shown in FIGS. 2C and 2D, respectively. The hysteresis loops in FIGS. 2C and 2D display dimensional changes in nanoporous gold as a function of the relative humidity. These hysteresis effects indicate that as the relative humidity increases during adsorption, the specimen shrinkage becomes more important beyond a certain threshold relative humidity (about 47% in FIG. 2C). Similarly, as the relative humidity decreases during desorption, the specimen expansion becomes more important below a certain threshold of relative humidity (about 41% in FIG. 2C). These two thresholds are consistent with the surface stress-induced nature of dimensional changes, since a significant change in surface stress only occurs if sufficient water molecules cover (upper threshold) or leave (lower threshold) the metal surface. These results indicate that the dimensional changes are directly associated to the amount of water vapour covering the gold-air interface.

Adsorption Induced Motions of a Cantilever

The physical adsorption-induced change in dimension was further explored to generate motions in a metallic cantilever. A 60 mm long and 20 μm thick cantilever was made from bilayer gold foils composed of a thick nanoporous gold layer (relative density 15%) and a thin pure gold layer. By bringing the cantilever from the ambient air relative humidity (about 45%) to a lower relative humidity (about 25%), motions of the cantilever can be visualised. Cantilever motions were also observed upon exposure to polar organic vapours, namely methanol and ethanol, indicating that the invention can also be used to detect solvent vapour contaminants.

Synthesis of a Nanoporous Au Device with Scale Morphology

A master alloy with composition Au₅Pd₂₀Ag₇₅ (atom %) was prepared by arc melting of pure Au (99.99%), Pd (99.99%) and Ag (99.999%). Rectangular samples (2×1×0.25 mm³) were cut from this master alloy for strain measurements and annealed at 350° C. for 48 hours to release stresses introduced during cold-rolling. Ag and Pd were selectively removed from these samples by free corrosion in 32% nitric acid, at room temperature. The free corrosion process was observed to be relatively slow, compared to the free corrosion of a Au—Ag alloy (about 20 times slower). At the end of the dealloying process, both gravimetric analysis and energy dissipative X-ray spectroscopy (ESD) were employed to verify that all Ag and Pd are etched away. A huge volume shrinkage was observed at the end the free corrosion process. The final volume of the dealloyed samples represented only a ˜20% volume fraction of the initial master alloy. This corresponds to 80% volume shrinkage. For comparison, 30% volume contraction has been reported for the conventional nanoporous Au obtained from the Au—Ag alloy. The thickness of the sample was mostly affected during the volume shrinkage; the decrease in thickness was about ˜72% (from ˜250 μm to ˜70 μm). The synthesised nanoporous Au samples had a porosity of about 79%. Note that the material also lost porosity due to the volume shrinkage. A porosity of 95% could have been achieved if no volume shrinkage occurred.

Microstructural Characterisation of the Scale Morphology Device

Ultra-high resolution scanning electron microscopy (UHR-SEM) Philips-XL30s SEM-FEG and high resolution transmission electron microscopy (HRTEM) Jeol-JEM-2010F, were employed to characterise the synthesised nanoporous Au scale morphology device.

Nanoporous Au obtained by free corrosion of Au15Ag85 (atom %) in nitric acid displays a homogenous morphology throughout its cross section, as shown by the scanning electron micrograph of FIG. 4A. In contrast, the scale morphology device displayed nanoporous Au layers throughout the entire bulk cross-section, parallel to the outer surface of the specimen, as shown in FIGS. 4B and 4C. SEM investigations of these layers reveal that at a given plane, each layer did not continuously cover the entire sample surface. It was observed as displayed in FIG. 5 (see also FIG. 10) that each layer was formed by scales, which are partially delaminated and also locally attached onto the underlying layers. The size of a scale varied between ˜10 μm and ˜200 μm. That the scales were indeed locally pinned onto other layers can be seen either at higher magnification (see FIGS. 4C and 5D) or at lower magnification if one compares the surface structure of an undeformed sample (see FIGS. 5A and 5B) with that of a mechanically deformed sample (see FIGS. 5C and 5D): when a sample is mechanically deformed, partial delamination of the scales was clearly visible at lower magnification as indicated by the arrows in FIGS. 5C and 5D (see also FIG. 10). The scales opened by bending-buckling because they are locally clamped onto the underlying layers.

UHR-SEM investigations indicated that the nanoporous Au structure had an average ligament diameter of ˜30 nm, as displayed in FIG. 6A. The expected secondary level of porosity was not observed in UHR-SEM analysis. Therefore, the structure was investigated using HRTEM. A single nanoporous Au ligament (see FIG. 6B) was observed to display ultra-fine porous structure with ligaments size of 1 nm, as shown in FIG. 6C-E. During HRTEM characterisations, these ultra-fine features were observed to be damaged by the exposure of the electron beam. This suggests that more damage of the ultra-fine structure might occur during TEM sample preparation (ion milling). An appropriate method to investigate the porosity without damaging the ultra-fine features is by using nitrogen adsorption (B.E.T).

Electrochemical Characterisation of the Scale Morphology Device

A three-electrode electrochemical cell containing 1 M perchloric acid, and a potentiostat (μAutolab III-FRA2, Eco Chemie) were used for the electrochemical characterisation. Successive cyclic voltammetry were performed in a potential range between 0.7 and 1.4 V with respect to Ag/AgCl reference electrode, at a scan rate of 10 mV·s⁻¹. During each scan, Au oxide layer was formed (anodic scan) and reduced (cathodic scan) at the Au-electrolyte interface. Evidence of Au Oxide layer formation and reduction can be seen on the TEM micrographs of FIG. 6B (oxidation) and on FIG. 7 (reduction), respectively. FIG. 6B shows that the nanoporous Au ligaments were covered with an oxide layer having an average thickness of 2 nm. In contrast, a sample subjected to cathodic scans did not display the Au oxide layer (FIG. 7).

Cyclic voltammograms obtained were conform to the well-known chemical signature of polycrystalline gold electrode as displayed in FIG. 8, where 21 successive cyclic voltammograms are plotted. From FIGS. 8A and 8B it was concluded that these 21 voltammograms are perfectly reproducible. Furthermore, the fact that no irregular peaks are observed, i.e. besides the expected Au oxidation and reduction peaks, provides supplementary confirmation that both Pd and Ag were fully etched away from the Au—Pd—Ag alloy.

Reversible Dimensional Changes of the Scale Morphology Device

Strain measurements were performed during similar cyclic voltammetry experiments as described above. A confocal displacement sensor (IFS2401-0.4 Micro-Epsilon) with a spot size of 10 μm and a resolution of 9 nm was used in situ to probe dimensional changes. Dimensional changes were measured in a direction perpendicular to the layers, and on different samples. Strain amplitudes achieved vary between 3 and 6%. The large amplitude curve in FIG. 9 displays a strain amplitude of ˜6% measured on a sample with scale morphology. If this is compared to the 0.1% strain amplitude measured on a nanoporous Au sample (see small amplitude curve in FIG. 9) it is concluded that the strain amplitude is boosted with a factor of almost two orders of magnitude due to the scale morphology. The origin of these giant strains is attributed to the deformation mechanism of the scales. First, expansion during opening of the scales contributes to these giant displacements. When the oxide layer is formed onto the ligaments as shown in FIG. 5B, nanoporous Au is under compressive surface stress. As results the bulk of the ligaments will elastically stretch under the compressive surface stress, in order to preserve mechanical equilibrium. Bulk expansion of the ligaments should normally result in a uniform volume expansion of the scales, for about 0.1-0.2%. However, because these scales are locally clamped onto other layers, their volume expansion is not longer uniform. Therefore, the scales can only deform by bending-buckling in a direction out of the layers planes. Due to the bending-buckling of the scales, the expected ˜0.1-0.2% lateral expansion of the sample's length (˜2 mm) and width (˜1 mm), which are up to 4 and 2 μm, respectively, are converted into out-of-plane displacements, i.e. the thickness of the sample (˜70 μm) expands with an amount of ˜2-4 μm. Strain amplitudes deduced from these out-of-plane displacements are comparable to the 3-6% measured during our experiments. A second deformation mechanism is contraction during closure of the scales. When the Au oxide layer is reduced (see supplementary FIG. 5) during the reverse cathodic scan, the compressive surface vanishes, the ligaments shrink back to their initial shapes, the bent scales relax and this gives rise to the macroscopic sample contraction.

Dimensional changes in the electrolyte are of the order of micrometers. Therefore, they should be visible in situ, in an optical microscope. This was verified on a thick nanoporous Au film with scale morphology. When the film edge was in focus, dimensional changes of the edge were observed. However, when focus was performed on the film surface, no lateral dimensional change of the film was observed. Instead, the surface appeared to get out of focus, which indicates out-of plane dimensional changes.

Device Response During Physical Adsorption/Desorption

Thick nanoporous Au films with scale morphology were observed to respond to changes in the relative humidity by bending. In extreme condition, nanoporous gold films with scale morphology rolled up into cylinders: a freshly dealloyed thick film did not roll up when kept at ambient conditions for drying. If the drying process was accelerated by exposing the film to a UV lamp, it rolled up into cylinder, due to the stress gradient which arises in the film during quick evaporation (the film side exposed to the lamp dries faster than the side in contact with the film holder (see FIG. 10). 

1. Method for creating a surface stress-induced volumetric change in a nanoporous material, comprising accumulating polar molecules onto the surface of the nanoporous material by physical adsorption of the polar molecules from the vapour phase thereby inducing surface stress to said surface; or dissipating accumulated polar molecules from the surface of the nanoporous material by physical desorption of the polar molecules into the vapour phase thereby inducing surface stress to said surface, wherein the nanoporous material comprises a nanoporous metal or alloy.
 2. Method according to claim 1, wherein said nanoporous material is a nanoporous metal or nanoporous alloy.
 3. Method according to claim 1, wherein said accumulating comprises the physical adsorption of at least a monolayer of the polar molecules onto the surface of the nanoporous material and wherein the accumulation preferably leads to the nanoporous material being filled with polar molecules to less than 10 vol. % of the total pore volume of the nanoporous material; or said dissipating comprises the physical desorption of at least a monolayer of accumulated polar molecules from the surface of the nanoporous material and wherein the dissipation preferably leads to the nanoporous material being depleted in polar molecules by less than 10 vol. % of the total pore volume of the nanoporous material.
 4. Method according to claim 2, wherein said nanoporous metal or alloy comprises a noble metal, preferably selected from the group consisting of Au, Pt, Pd and Ag, preferably the nanoporous material is nanoporous gold.
 5. Method according to claim 1, wherein said polar molecules comprise one or more selected from the group consisting of water, methanol and ethanol, more preferably said polar molecules comprise water molecules.
 6. Method according to claim 1, wherein the nanoporous material has a surface-to-volume ratio of 10 m²/cm³ or more, preferably 25 m²/cm³ or more, more preferably in the range of 25-500 m²/cm³.
 7. Method according to claim 1, wherein the nanoporous material has a porosity of 0.3 or more, preferably in the range of 0.3-0.95.
 8. Method according to claim 1, wherein the nanoporous material is a material wherein the pores have an average cross-sectional diameter of 500 nm or less, preferably 100 nm or less, more preferably in the range of 2-60 nm.
 9. Method according to claim 1, wherein the physical adsorption or desorption is the result of a change in vapour pressure of the polar molecules in the environment, preferably the physical adsorption or desorption is the result of a change in relative humidity of the environment.
 10. Method according to claim 1, wherein said volumetric change is at least partially reversible.
 11. Method according to claim 1, wherein the volumetric change has a size and/or a sign that is/are dependent on the nature, such as the polarity sign, of the polar molecules; and/or the state, such as the cleanness, of the surface of the nanoporous material.
 12. Method according to claim 1, wherein said volumetric change is characterised by a strain amplitude of 0.005% or more, preferably 0.01% or more in response to a change in vapour pressure of the polar molecules in the environment of 15%.
 13. Device for carrying out the method of claim 1, comprising a) a nanoporous metal material which, upon physical adsorption from the vapour phase or physical desorption into the vapour phase of polar molecules onto or from the surface of the nanoporous metal material, exhibits a volumetric change; and b) a mechanism for detecting and/or transferring the volumetric change.
 14. Device according to claim 13, wherein the nanoporous metal or alloy material has a layered structure wherein each layer comprises a multitude of scales, and wherein at least a part of the scales in each layer are locally attached to one or more scales of an adjacent layer.
 15. Device according to claim 13 in the form of a sensing device or actuating device.
 16. Device according to claim 13, which device is operative in the absence of electricity, heat, and chemical reaction energy external stimuli. 